1. Field of the Invention
The present invention relates to an infrared detector using a pyroelectric infrared detection element.
2. Description of the Related Art
FIG. 2 illustrates a circuit diagram of a related-art infrared detector using a pyroelectric infrared detection element.
The related-art infrared detector using a pyroelectric infrared detection element includes a pyroelectric infrared detection element 100, resistors 107 and 109, a constant voltage circuit 200, and an output terminal 110.
The pyroelectric infrared detection element 100 includes a pyroelectric element 101, an NMOS transistor 102, and a resistor 103.
The NMOS transistor 102 has a gate connected to the pyroelectric element 101 and the resistor 103, a drain connected to the output terminal 110 and to the constant voltage circuit 200 via the resistor 109, and a source connected to GND via the resistor 107. In other words, an output stage of the pyroelectric infrared detection element 100 serves as a common source amplifier circuit of the NMOS transistor 102.
The above-mentioned pyroelectric infrared detection element 100 operates as follows to detect infrared ray.
When infrared ray enters the pyroelectric infrared detection element 100, the resistor 103 converts electric charges generated in the pyroelectric element 101 into a voltage, and then a gate voltage of the NMOS transistor 102 increases. When the gate voltage increases, a drain current flows through the NMOS transistor 102 to vary a voltage at a node between the drain of the NMOS transistor 102 and the resistor 109, that is, a voltage at the output terminal 110. In this manner, because the NMOS transistor 102 serves as a common source amplifier circuit, by setting an appropriate resistance value of the resistor 109, the output stage of the pyroelectric infrared detection element 100 can obtain a desired amplification factor (see, for example, Japanese Patent Application Laid-open No. Hei 05-340807).
In the output stage of the above-mentioned infrared detector, the amplification factor is determined by the ratio of the resistance values of the resistor 107 and the resistor 109. In other words, in order to increase the amplification factor, it is necessary to increase the resistance value of the resistor 109 to be larger than that of the resistor 107. Accordingly, if the amplification factor is increased, a higher voltage is generated in the resistor 109, and hence it is necessary to increase a voltage to be output from the constant voltage circuit 200. In other words, the related-art infrared detector has a problem in that low voltage operation cannot be performed.